The present invention relates to an input element for use as a transducer in keyboard composed of a piezoelectric material, preferably a sheet-like polymeric piezoelectric material.
Piezoelectric materials are divided into two types, namely inorganic materials such as barium titanate or quartz and polymeric materials. The piezoelectric materials of the latter type are excellent in the shock resistance and water resistance and are advantageous over the piezoelectric materials of the former type in that sheets having a large area and being uniform in the piezoelectric characteristics can easily be obtained. Accordingly, by utilizing these advantageous characteristics, a keyboard having a high reliability can be obtained by a low cost by forming a plurality of stimulus-receiving portions such as pressure-sensitive portions (input portions and detecting portions) on one sheet composed of a polymeric piezoelectric material.
As means for detecting mechanical stimula imposed on a piezoelectric sheet composed of a polymeric piezoelectric material in the form of electric signals, there have heretofore been proposed the following three methods.
The first method uses two piezoelectric films. More specifically, as illustrated in FIG. 1, on opposite surfaces of two piezoelectric films 1a and 1b there are disposed electrodes 2 as the X-coordinate signal and electrodes 3 as the Y-coordinate signal, respectively, and the two films 1a and 1b are piled with a sheet-like earthing electrode 4 being interposed therebetween so that the electrodes 2 and 3 intersect each other at a right angle, and plastic plates 5 and 7 having key holes 6 and 8 formed thereon are laminated on the above assembly. According to this conventional technique, since two piezoelectric films are piled, the flexibility inherent of the polymeric piezoelectric material is lost and there is produced a considerable difference of the output level between the upper and lower piezoelectric films. There are observed such defects as reduction of the magnitude of the output signal and indispensable increase of the amount used of the piezoelectric material.
According to the second conventional method, as shown in FIG. 2, electrodes 12 and 13 for detection in the X-direction and the Y-direction are formed in regions corresponding to the respective coordinates (keys) on one piezoelectric film 11, and one electrode 13 is connected to a Y-direction conduction circuit through a through hole 17 piercing a substrate 18 of the piezoelectric film. In this method, however, electric connection of the electrode 13 on the piezoelectric film 11 with the through hole 17 of the substrate 18 is very difficult, and the reliability of this connection is very low. Furthermore, an excessive area is necessary for the through hole 17 and, therefore, this method is defective in that the area density of coordinates or keys cannot be sufficiently increased.
According to the third conventional method, as shown in FIG. 3, electrodes 21 and 22 for the X-direction and the Y-direction are formed on the upper and lower surfaces of a piezoelectric film 20, respectively, and signals of respective coordinates or keys are detected by the dynamic drive system. In this conventional method, however, since no complete earthing is attainable, when a stimulus is given to a certain coordinate point, other undesirable output signals appear on other coordinate points through capacitive coupling, that is so-called cross-talk occurs. The maximum magnitude of this cross-talk is as high as 1/2 of the magnitude of the desired signal output from the coordinate point given the stimulus positively. Therefore, the S/N ratio of the desired signal is low. This is a fatal defect to a coordinate input device or keyboard, and the product according to this conventional method cannot be put into practical use.